1. Field
This invention pertains generally to spent fuel pools in nuclear power plants and, more particularly, to systems and methods for measuring and monitoring axial flux to evaluate subcriticality in a spent fuel pool.
2. Description of Related Art
The generation of electric power in a nuclear power plant is accomplished by the nuclear fission of radioactive materials. Due to the volatility of the nuclear reaction, nuclear power plants are required by practice to be designed in such a manner that the health and safety of the public is assured.
In conventional nuclear power plants used for generating electric power, the nuclear fuel becomes spent and is removed at periodic intervals from the nuclear reactor and replaced with fresh fuel. The spent fuel generates decay heat and remains radioactive after it has been removed from the nuclear reactor. Thus, a safe storage facility is provided to receive the spent fuel. In nuclear reactors, such as pressurized water reactors, a pool is provided as a storage pool for the spent fuel. The spent fuel pool is designed to contain a level of water such that the spent fuel is stored underwater. The spent fuel pool is typically constructed of concrete and is at least 40 feet deep. In addition to the level of the water being controlled and monitored, the quality of the water is also controlled and monitored to prevent fuel degradation when it is in the spent fuel pool. Further, the water in the spent fuel pool is continuously cooled to remove the heat which is produced by the spent fuel.
A spent fuel pool in a nuclear power plant typically consists of more than several hundred fuel assembly storage racks filled with either depleted or fresh fuel assemblies. Reactivity of the pool is expressed by a neutron effective multiplication factor, k-effective. The value of k-effective is typically determined by analytical means, such as by the use of Monte Carlo simulations.
Known storage configurations in the spent fuel pool can include close-packed, checker-boarding with empty water cells, and with or without neutron absorbers. The selected storage configuration depends on the reactivity of the depleted assemblies. The storage configuration is selected to ensure that the overall reactivity of the pool remains below regulatory limits.
Monitoring and controlling the margin of subcriticality in the spent fuel pool can assure safe operation of the pool. It is known to obtain this information by means of analytical methods which are based on conservative input assumptions to encompass a wide range of core operating parameters for depleted fuel assemblies. As a result, a considerable amount of subcritical margin may exist in the spent fuel pool based on the analytical results.
Due to a lack of reprocessing and a shortage of permanent disposal sites, commercial nuclear utilities are interested in systems and methods to increase storage capability as some nuclear power plants operate near full capacity in the spent fuel pool. Higher initial enrichments in close-packed storage configurations and degradation problems with reactivity control materials are a couple of the factors which compound the uncertainty associated with pool reactivity and therefore, creates regulatory concerns over the safe operation of spent fuel pools.
Thus, there is a desire in the nuclear power industry to develop a system and method for measuring k-effective with increased certainty and decreased margin so as to achieve at least one of the following benefits: (1) an increase in the amount of soluble boron that can be credited and thereby effectively increasing the storage capacity, reducing the number of different and complex storage configurations, and simplifying the technical specification compliance, and (2) eliminate the regulatory concerns on the uncertainties as to whether there is enough margin to criticality or whether the regulatory limits are satisfied.
This invention addresses the issues above-described by providing systems and methods for measuring and monitoring the margin of subcriticality in the spent fuel pool which is based on measuring axial flux in the spent fuel pool, generating an axial flux curve, and correlating the curve with analytical data to determine the k-effective and monitor any reactivity changes, e.g., inadvertent and anticipated, in the spent fuel pool.